An in-line turboprop engine generally includes in serial alignment in the direction of the flow through the propulsion system, a propeller, an air intake and an engine arranged in flow communication with one another in succession. Thus, the air intake is disposed downstream of the propeller, and the engine is disposed downstream of the air intake in the direction of the flow through the engine. The gas turbine engine generally includes, in serial fluid flow order, a compressor section, a combustion section, a high pressure turbine section, a power turbine section and an exhaust section. Air is drawn into the air intake and compressed by the compressor in the compressor section. Fuel is added in the combustion section, and the fuel-air mixture is combusted in the combustion section to produce hot combustion gases that expand through the high pressure and power turbine sections and produce rotational mechanical energy that can be tapped to drive one or more shafts that are coupled to the power turbine. In order of succession, there is the propeller at the very front of the engine, followed by the propeller gearbox, followed by the exhaust, followed by the power turbine, followed by the high pressure turbine, followed by the compressor and followed by the air inlet, which is at the aft end of the engine.
With multi-shaft gas turbine engines, a compressor shaft that is driven by the high pressure turbine is used to drive the compressor in the compressor section. A propeller shaft that is driven by the power turbine is used to provide rotational input to a reduction gear that converts the high revolutions per minute but low torque rotation of the power turbine into a low rotation per minute but high torque rotation that is used to drive the propeller. The reduction gear can be housed in a propeller gearbox of an epicyclic gearing arrangement, which allows the propeller to be rotated at fewer revolutions per unit of time than the rotational speed of the power turbine for greater efficiency. The propeller gearbox rotatably supports a sun gear that is disposed centrally with respect to a ring gear and a plurality of planet gears, which are disposed around the sun gear and engage between the sun gear and the ring gear. The power turbine shaft provides the input to the epicyclic gearing arrangement by being coupled to the sun gear, while the propeller can be coupled in line with the power turbine shaft to rotate in unison with the carrier of the planet gears or with the ring gear, depending upon whether a star gearbox or a planetary gearbox is used. Each planet gear meshes with the sun gear and with the ring gear. One of the carrier or the ring gear may be held stationary, but not both of them. Each planet gear is rotatable on its own bearing that is mounted on a support pin housed within a epicyclic gearing arrangement, which is fixed to the peripheral region of the carrier of the epicyclic gearing arrangement.
When the aircraft is on the ground, electric power nonetheless must be generated to operate various electrical systems on the aircraft. This auxiliary electric power can be provided by an auxiliary power unit (APU). A portable gas powered electric generator can be moved from a ground storage facility to the aircraft and connected to the electrical system of the aircraft when the aircraft is on the ground and requires this auxiliary power to run its electrical systems in the absence of operation of the turboprop engine. However, this solution has operational costs that stem from the need to maintain the equipment on the ground and competent personnel to connect it and disconnect it each time the aircraft requires electrical power when on the ground. Accordingly, some aircraft carry on board a small gas turbine engine that can be operated to provide the auxiliary electric (or hydraulic) power, but this solution has the drawback of the increased cost of the engine and the additional weight that must be carried by the aircraft even when the engine is not needed. While this auxiliary power can be provided by aircraft's own turboprop engine, a rotating propeller during operation of the aircraft engine to generate the auxiliary power when the aircraft is on the ground becomes problematical due to concerns for the safety of passengers and ground crew who could be seriously injured or killed by contact with the rotating propeller.